1. Field of the Invention
This invention relates to coated cemented carbides excellent in wear resistance and toughness and more particularly, it is concerned with multilayer coated cemented carbides suitable for cutting tools which can be used at a high cutting speed, e.g. exceeding 300 m/min for a long period of time.
2. Description of the Prior Art
Cemented carbides coated with one or more layers consisting essentially of at least one of hard materials such as TiC, Al.sub.2 O.sub.3, TiN, etc. excellent in wear resistance have hitherto been put to practical use. Above all, it is well known that double layer coated cemented carbide tools, whose outer layer is of Al.sub.2 O.sub.3, have an excellent wear resistance because of the higher heat resistance and oxidation resistance of Al.sub.2 O.sub.3. Cutting conditions required for such cutting tools have yearly become severer and the requirement of a higher wear resistance for the cutting tools is increasing with the increase of cutting speeds, e.g. exceeding 300 m/min. For this requirement, an effort has been made to increase the thickness of an Al.sub.2 O.sub.3 layer.
However, Al.sub.2 O.sub.3 itself is a brittle material, so the increase of its film thickness often results in lowering of the toughness and accordingly, a cutting tool using a cemented carbide coated with an increased thickness of Al.sub.2 O.sub.3 layer is lacking in toughness so as not to be satisfactory as a widely available tool possessing both the wear resistance and toughness.